1. Field of the Invention
This invention relates to a heat-sensitive recording sheet having excellent oil resistance and preservation stability.
2. Prior Art
Heat-sensitive recording sheets utilizing the color development reaction of a so-called basic colorless dyestuff which is in general colorless or light color with an organic color-developer such as phenols and organic acids by heating are disclosed in Japanese Patent Publication Nos. 4160/1968 and 14039/1970, Japanese Laid-Open Patent Application Nos. 277361/1973 and the like, and these heat-sensitive recording sheets have been widely applied for practical use.
Generally, a heat-sensitive recording sheet is produced by applying on the substrat, such as paper and film, the coating which is prepared by individually grinding and dispersing a basic colorless dyestuff and an organic color-developer into fine particles, mixing the resultant dispersion with each other and then adding thereto binder, filler, sensitizer, slipping agent and other auxiliaries. When this sheet is heated, the coating undergoes instantaneously a chemical reaction which develops a color. In this case various bright colors can be advantageously formed depending upon selection of specific colorless dyestuff.
These heat-sensitive recording sheets have now been applied in wide range of fields, such as recorders for measurement in the medical field or industrial field, terminal printer of computers and information communication systems, printers of electric desk computers, automatic ticket vending machine and the like. The heat-sensitive recording papers afford a clear records of high density, cause no troubles such as adhesion of refuse to the thermal head and sticking, and possess excellent recording property, but, on the other hand, the improvement of their basic qualities, such as less color development of ground color with the passage of time, has been required.
The present applicants already revealed that a heat-sensitive recording paper using 4-isopropoxy-4'-hydroxy diphenylsulfone or 4-methyl-4'-hydroxy diphenylsulfone as a developer is quite excellent in the basic qualities described above in Japanese Patent Application No. 175374/1984 or 96691/1984.
Meanwhile, heat-sensitive recording papers are inevitably touched with the hands of man, in view of their function as papers to record information. As the fingers of the operator are often adhered by oily substances such as his hair tonic daily used and oils contained in the sweat on his skin, it may safely be said that the heat-sensitive recording papers are most frequently contaminated by such oily substances. In general, the heat-sensitive recording papers have insufficient stability against these oily substances, so that the density of the developed color image on the contaminated part is often reduced or disappeared. Also, the phenomenon of discoloration of the contaminated white ground is observed. Their causes can not be sufficiently elucidated yet, but it is supposed that the oily substances partly dissolve the coloring layer consisting of the fine granular basic colorless dyestuff and organic developer or coloring reactant thereof, or make the coloring layer or coloring reactant thereof unstable.
As described above, the heat-sensitive recording paper containing 4-isopropoxy-4'-hydroxy diphenylsulfone or 4-methyl-4'-hydroxy diphenylsulfone as the developer has excellent basic qualities, while its stability against the oily substances was found to be slightly poor, compared with bisphenol-type developers generally used conventionally.
As to the bisphenol-type developer, Japanese Patent Application OPI No. 6795/1982 proposes a method to improve preservation stability such as plasticizer resistance by using a metal salt of aliphatic carboxylic acids or a metal salt of aromatic carboxylic acids together with the developer. However, the combination of the metal salts of these carboxylic acids with the heat-sensitive recording paper containing 4-isopropoxy-4'-hydroxy diphenylsulfone or 4-methyl-4'-hydroxy diphenylsulfone as the developer hardly improve the stability against the oily substances, and the reduction of preservation stability of ground color and developed images was instead observed.